If you have already added your GigSky SIM card to a GigSky account, you will not be able to buy data plans using the Paid Wi-Fi & Cellular app. You must use your GigSky account to buy plans, either with the GigSky smartphone app, or on the web, at app.gigsky.com.
If your SIM card has not yet been used, please contact Microsoft Support directly. We are unable to assist with any issues related to the Microsoft Store. The Microsoft Store Support page is here: https://support.microsoft.com/en-in/help/28808/store-contact-support